Method For Producing Super Absorbent Polymer

ABSTRACT

A super absorbent polymer produced by the preparation method of the super absorbent polymer according to the present invention has excellent dryness while maintaining excellent absorption performance, and thus is preferably used for hygienic materials such as diapers and can exhibit excellent performance.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of Korean PatentApplication No. 10-2016-0173803 filed on Dec. 19, 2016 with the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for producing a superabsorbent polymer having excellent dryness while maintaining excellentabsorption performance.

BACKGROUND ART

Super absorbent polymer (SAP) is a synthetic polymer material capable ofabsorbing moisture from about 500 to about 1,000 times its own weight,and each manufacturer has denominated it as different names such as SAM(Super Absorbency Material), AGM (Absorbent Gel Material) or the like.Such super absorbent polymers started to be practically applied insanitary products, and now they are widely used for preparation ofhygiene products such as paper diapers for children or sanitary napkins,water retaining soil products for gardening, water stop materials forthe civil engineering and construction, sheets for raising seedling,fresh-keeping agents for food distribution fields, materials forpoultice or the like.

In most cases, these super absorbent polymers have been widely used inthe field of hygienic materials such as diapers or sanitary napkins. Insuch hygienic materials, the super absorbent polymer is generallycontained in a state of being spread in the pulp. In recent years,however, continuous efforts have been made to provide hygienic materialssuch as diapers having a thinner thickness. As a part of such efforts,the development of so-called pulpless diapers and the like in which thecontent of pulp is reduced or pulp is not used at all is being activelyadvanced.

As described above, in the case of hygienic materials in which the pulpcontent is reduced or the pulp is not used, a super absorbent polymer iscontained at a relatively high ratio and these super absorbent polymerparticles are inevitably contained in multiple layers in the hygienicmaterials. In order for the whole super absorbent polymer particlescontained in the multiple layers to absorb liquid such as urine moreefficiently, it is necessary for the super absorbent polymer tobasically exhibit high absorption performance and absorption rate.

For this purpose, the conventional super absorbent polymer uses a methodof lowering the degree of internal crosslinking and increasing thedegree of surface crosslinking. According to this method, however, theabsorption rate may be increased, but after the super absorbent polymeris swollen by the absorbed liquid, the liquid is present on the surfaceof the super absorbent polymer, which causes a decrease in wearingfeeling, a skin rash or the like.

As described above, the extent to which no liquid is present on thesurface after the super absorbent polymer absorbs the liquid is referredto as “dryness”. Therefore, there is a need to develop a super absorbentpolymer having excellent dryness, without impairing the absorptionperformance and absorption rate of the super absorbent polymer.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

It is an object of the present invention to provide a method forproducing a super absorbent polymer having excellent dryness whilemaintaining excellent absorption performance, and a super absorbentpolymer produced thereby.

Technical Solution

In order to achieve the above objects, the present invention provides amethod for producing a super absorbent polymer as follows:

the method for producing a super absorbent polymer comprising the stepsof:

crosslinking a water-soluble ethylenically unsaturated monomer having atleast partially neutralized acidic groups in the presence of an internalcrosslinking agent and a thermal polymerization initiator (step 1);

drying, pulverizing and classifying the hydrogel polymer to form a basepolymer power (step 2); and

heat-treating and surface-crosslinking the base polymer powder in thepresence of a surface crosslinking solution to form a super absorbentpolymer particle (step 3),

wherein the thermal polymerization initiator is used in an amount of0.04 to 0.22 parts by weight based on 100 parts by weight of theethylenically unsaturated monomer.

The method for producing a super absorbent polymer according to thepresent invention is intended to produce a super absorbent polymerhaving excellent dryness while maintaining excellent absorptionperformance, and is characterized by adjusting the content of thethermal polymerization initiator especially in the production of thehydrogel polymer. The characteristics of the base polymer powderprepared in step 2 can be controlled according to the content of thethermal polymerization initiator. This surface crosslinking makes itpossible to produce a super absorbent polymer having excellent drynesswhile having excellent absorption rate.

Hereinafter, embodiments of the present invention will be described inmore detail.

(Step 1)

Step 1 is a step of forming a hydrogel polymer which is a step ofcrosslinking a monomer composition comprising an internal crosslinkingagent, a thermal polymerization initiator, and a water-solubleethylenically unsaturated monomer having at least partially neutralizedacidic groups.

The water-soluble ethylenically unsaturated monomer constituting thefirst cross-linked polymer may be any monomer commonly used in theproduction of a super absorbent polymer. As a non-limiting example, thewater-soluble ethylenically unsaturated monomer may be a compoundrepresented by the following Chemical Formula 1:

R₁—COOM¹   [Chemical Formula 1]

in Chemical Formula 1,

R₁ is an alkyl group having 2 to 5 carbon atoms containing anunsaturated bond, and

M is a hydrogen atom, a monovalent or divalent metal, an ammonium groupor an organic amine salt.

Preferably, the above-described monomer may be at least one selectedfrom the group consisting of acrylic acid, methacrylic acid, and amonovalent metal salt, a divalent metal salt, an ammonium salt, and anorganic amine salt thereof. When acrylic acid or a salt thereof is usedas the water-soluble ethylenically unsaturated monomer, it isadvantageous in that a super absorbent polymer having improvedabsorption property can be obtained. In addition, as the monomer, maleicanhydride, fumaric acid, crotonic acid, itaconic acid,2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid,2-(meth)acryloylpropane sulfonic acid, or2-(meth)acrylamido-2-methylpropane sulfonic acid, (meth)acrylamide,N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylate,polyethylene glycol (meth)acrylate,(N,N)-dimethylaminoethyl(meth)acrylate,(N,N)-dimethylaminopropyl(meth)acrylamide, and the like may be used.

Here, the water-soluble ethylenically unsaturated monomers may have anacidic group, wherein at least a part of the acidic group may beneutralized. Preferably, the monomers may be those partially neutralizedwith an alkali substance such as sodium hydroxide, potassium hydroxide,ammonium hydroxide, or the like.

In this case, a degree of neutralization of the monomer may be 40 to 95mol %, or 40 to 80 mol %, or 45 to 75 mol %. The range of the degree ofneutralization may vary depending on the final physical properties.However, an excessively high degree of neutralization causes theneutralized monomers to be precipitated, and thus polymerization may notreadily occur, whereas an excessively low degree of neutralization notonly greatly deteriorates the absorbency of the polymer, but also endowsthe polymer with hard-to-handle properties, such as those of an elasticrubber.

Further, the concentration of the water-soluble ethylenicallyunsaturated monomer in the monomer composition may be appropriatelyadjusted in consideration of the polymerization time, the reactionconditions and the like, and it may be preferably 20 to 90% by weight,or 40 to 65% by weight. These concentration ranges may be advantageousfor adjusting the pulverization efficiency during pulverization of thepolymer described below, without needing to remove unreacted monomersafter polymerization by using the phenomenon of gel effect occurring inthe polymerization reaction of the highly concentrated aqueous solution.However, when the concentration of the monomer is excessively low, theyield of the super absorbent polymer can be lowered. Conversely, whenthe concentration of the monomer is excessively high, it may ariseproblems in the processes, for example, a part of the monomer may beprecipitated, or the pulverization efficiency may be lowered duringpulverization of the polymerized hydrogel polymer, etc., and thephysical properties of the super absorbent polymer may be deteriorated.

Meanwhile, as the thermal polymerization initiator, one or more selectedfrom the group consisting of a persulfate-based initiator, an azo-basedinitiator, hydrogen peroxide, and ascorbic acid may be used. Specificexamples of the persulfate-based initiator may include sodium persulfate(Na₂S₂O₈), potassium persulfate (K₂S₂O₈), ammonium persulfate((NH₄)₂S₂O₈), and the like; and examples of the azo-based initiatorinclude 2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutylonitril,2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,4,4-azobis-(4-cyanovaleric acid) or the like.

In particular, in the present invention, the thermal polymerizationinitiator is used in an amount of 0.04 to 0.22 parts by weight based on100 parts by weight of the ethylenically unsaturated monomer. The amountof the thermal polymerization initiator used affects the physicalproperties of the base polymer prepared in step 2, and particularlyaffects the extractable content of the base polymer. If the extractablecontent is increased, the physical properties of the super absorbentpolymer produced in step 3 are deteriorated, and in particular, the6-hour dryness is deteriorated. Thus, in the present invention, a superabsorbent polymer having excellent dryness while maintaining excellentabsorption performance is produced by using the above-mentioned contentof the thermal polymerization initiator. When the amount of the thermalpolymerization initiator used exceeds 0.22 parts by weight, the 6-hourdryness of the finally produced super absorbent polymer is lowered.Further, when the amount of the thermal polymerization initiator used isless than 0.04 parts by weight, the efficiency of the hydrogelpolymerization decreases and various physical properties of the finallyproduced super absorbent polymer decreases. Preferably, the thermalpolymerization initiator is used in an amount of 0.05 to 0.20 parts byweight based on 100 parts by weight of the ethylenically unsaturatedmonomer.

Further, step 1 can be carried out in the presence of an internalcrosslinking agent. As the internal crosslinking agent, any compound canbe used without limitation as long as it enables introduction of acrosslink bond upon polymerization of the water-soluble ethylenicallyunsaturated monomer. Non-limiting examples of the internal crosslinkingagent may include multifunctional crosslinking agents, such asN,N′-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate,ethylene glycol di(meth)acrylate, polyethylene glycol(meth)acrylate,propylene glycol di(meth)acrylate, polypropylene glycol(meth)acrylate,butanediol di(meth)acrylate, butylene glycol di(meth)acrylate,diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate,triethylene glycol di(meth)acrylate, tripropylene glycoldi(meth)acrylate, tetraethylene glycol di(meth)acrylate,dipentaerythritol pentacrylate, glycerin tri(meth)acrylate,pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidylether, propylene glycol, glycerin, or ethylene carbonate, which may beused alone or in combination of two or more thereof, but are not limitedthereto. Preferably, two kinds of polyethylene glycol diacrylates havingdifferent molecular weights are used

Such internal crosslinking agent may be added at a concentration of0.001 to 1% by weight, based on the monomer composition. That is, if theconcentration of the internal crosslinking agent is too low, theabsorption rate of the polymer is lowered and the gel strength maybecome weak, which is undesirable. Conversely, if the concentration ofthe internal crosslinking agent is too high, the absorption capacity ofthe polymer is lowered and thereby is not preferred for an absorbent.

Further, step 1 can be carried out in the presence of a surfactant. Thesurfactant serves to uniformly distribute bubbles in the entire area ofthe polymer simultaneously while maintaining the shape of bubbles formedduring polymerization, which serves to increase the surface area of thepolymer. Preferably, as the surfactant, a cationic surfactant, ananionic surfactant, or a nonionic surfactant can be used. As an example,as the anionic surfactant, a compound represented by the followingChemical Formula 2 can be used, and more preferably, sodium dodecylsulfate can be used.

R—SO₃Na   [Chemical Formula 2]

in Chemical Formula 2,

R is an alkyl having 8 to 16 carbon atoms.

Further, the surfactant is preferably used in an amount of 0.001 to 1%by weight based on the weight of the water-soluble ethylenicallyunsaturated monomer. When the amount of the surfactant used exceeds 1%by weight, no substantial improvement effect is obtained, and thecontent of the surfactant in the super absorbent polymer increases,which is not preferable.

Further, step 1 can be carried out in the presence of a foaming agent.The foaming agent acts to cause causing foaming during polymerization toproduce pores in the hydrogel polymer, thereby increasing the surfacearea. As the foaming agent, an inorganic foaming agent or an organicfoaming agent can be used. Examples of the inorganic foaming agent mayinclude sodium bicarbonate, sodium carbonate, potassium bicarbonate,potassium carbonate, calcium bicarbonate, calcium carbonate, magnesiumbicarbonate or magnesium carbonate. Further, examples of the organicfoaming agent may include azodicarbonamide (ADCA), dinitrosopentamethylene tetramine (DPT), p,p′-oxybisbenzene sulfonylhydrazide(OBSH), and p-toluenesulfonyl hydrazide (TSH).

Further, the foaming agent is preferably used in an amount of 0.01 to 1%by weight based on the weight of the water-soluble ethylenicallyunsaturated monomer. When the amount of the foaming agent used exceeds1% by weight, the pores become too large, the gel strength of the superabsorbent polymer lowers and the density becomes low, which may causeproblems in distribution and storage.

In addition, the monomer composition may further comprise additives suchas a thickener, a plasticizer, a preservation stabilizer, anantioxidant, etc., if necessary.

Such monomer composition may be prepared in the form of a solution inwhich raw materials such as the above-mentioned monomer are dissolved ina solvent. In this case, any usable solvent can be used withoutlimitation in the constitution as long as it can dissolve theabove-mentioned raw materials. Examples of the solvent may includewater, ethanol, ethylene glycol, diethylene glycol, triethylene glycol,1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether,propylene glycol monomethyl ether, propylene glycol monomethyl etheracetate, methyl ethyl ketone, acetone, methyl amyl ketone,cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether,diethylene glycol ethylether, toluene, xylene, butyrolactone, carbitol,methyl cellosolve acetate, N,N-dimethylacetamide, or a mixture thereof.

Further, the formation of the hydrogel polymer through polymerization ofthe monomer composition may be performed by a general polymerizationmethod. For example, the polymerization can be carried out in a reactorlike a kneader equipped with agitating spindles. In this case, themonomer composition is injected into a reactor like a kneader equippedwith the agitating spindles, and thermal polymerization can be performedby providing hot air thereto or heating the reactor, thereby obtainingthe hydrogel polymer. At this time, the hydrogel polymer, which isdischarged from the outlet of the reactor according to the type ofagitating spindles equipped in the reactor, can be obtained as particleswith a size of centimeters or millimeters. Specifically, the hydrogelpolymer may be obtained in various forms according to the concentrationof the monomer composition injected thereto, the injection speed, or thelike, and the hydrogel polymer having a (weight average) particlediameter of 2 to 50 mm may be generally obtained.

Meanwhile, the hydrogel polymer obtained by the above-mentioned methodmay have a water content of 40 to 80% by weight. Meanwhile, the “watercontent” as used herein means a weight occupied by moisture with respectto a total weight of the hydrogel polymer, which may be the valueobtained by subtracting the weight of the dried polymer from the weightof the hydrogel polymer. Specifically, the water content can be definedas a value calculated by measuring the weight loss due to evaporation ofmoisture in the polymer in the drying process by raising the temperatureof the polymer through infrared heating. At this time, the dryingconditions may be determined as follows: the drying temperature isincreased from room temperature to about 180° C. and then thetemperature may be maintained at 180° C., and the total drying time maybe set to 20 minutes, including 5 minutes for the temperature risingstep.

(Step 2)

Step 2 is a step of drying, pulverizing and classifying the hydrogelpolymer prepared in step 1 to form a base polymer power, and the basepolymer powder and the super absorbent polymer obtained therefrom aresuitably prepared and provided so as to have a particle size of 150 to850 μm. More specifically, at least 95% by weight of the base polymerpowder and the super absorbent polymer obtained therefrom have aparticle size of 150 to 850 μm, and fine powders having a particle sizeof less than 150 μm can be less than 3% by weight. As the particle sizedistribution of the base polymer powder and the super absorbent polymeris adjusted within the preferable range as described above, the superabsorbent polymer finally produced can already exhibit theabove-mentioned physical properties more satisfactorily.

Meanwhile, the method of proceeding the drying, grinding and classifyingwill be described in snore detail below.

First, when drying the hydrogel polymer, a step of coarse pulverizationmay be further carried out before drying in order to increase theefficiency of the drying step, if necessary. A pulverizing machine usedherein may include, but its configuration is not limited to, forexample, any one selected from the group consisting of a verticalpulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, acutter mill, a disc mill, a shred crusher, a crusher, a chopper, and adisc cutter. However, it is not limited to the above-described examples.

In this case, the coarse pulverizing step may be performed so that thehydrogel polymer has a particle size of about 2 mm to about 10 mm.Pulverizing the hydrogel polymer into a particle size of less than 2 mmis technically not easy due to a high water content of the hydrogelpolymer, and a phenomenon of agglomeration may occur between thepulverized particles. Meanwhile, if the hydrogel polymer is pulverizedinto a particle size of larger than 10 mm, the effect of increasing theefficiency in the subsequent drying step may be insignificant.

The hydrogel polymer coarsely pulverized as above or immediately afterpolymerization without the coarsely pulverizing step is subjected to adrying step.

At this time, the drying temperature may be 50 to 250° C. When thedrying temperature is less than 50° C., it is likely that the dryingtime becomes too long or the physical properties of the super absorbentpolymer finally formed is deteriorated. When the drying temperature ishigher than 250° C., only the surface of the polymer is excessivelydried, and thus fine powder may be generated during the subsequentpulverization process and the physical properties of the super absorbentpolymer finally formed may be deteriorated. More preferably, the dryingmay be performed at a temperature of 150 to 200° C., and more preferablyat a temperature of 160 to 190° C. Meanwhile, the drying step may becarried out for 20 minutes to 15 hours, in consideration of the processefficiency, but is not limited thereto.

In the drying step, any drying method may be selected and used withoutlimitation in the constitution if it is a method commonly used in therelevant art. Specifically, the drying step may be carried out by amethod such as hot air supply, infrared irradiation, microwaveirradiation or ultraviolet irradiation. When the drying step as above isfinished, the water content of the polymer may be 0.05 to 10% by weight.

Next, a step of pulverizing the dried polymer obtained through such adrying step is carried out. The polymer powder obtained through thepulverizing step may have a particle diameter of 150 μm to 850 μm.Specific examples of a pulverizing device that can be used to pulverizeinto the above particle diameter may include a ball mill, a pin mill, ahammer mill, a screw mill, a roll mill, a disc mill, a jog mill or thelike, but it is not limited to the above-described examples.

Further, in order to control the physical properties of the superabsorbent polymer powder finally commercialized after the pulverizationstep, a separate step of classifying the polymer powder obtained afterthe pulverization depending on the particle diameter may be undergone.Preferably, a polymer having a particle diameter of 150 μm to 850 μm isclassified and only the polymer powder having such a particle diameteris subjected to the surface crosslinking reaction described later andfinally commercialized.

(Step 3)

Step 3 is a step of crosslinking the surface of the base polymerprepared in step 2, which is a step of hear-treating andsurface-crosslinking the base polymer powder in the presence of asurface crosslinking solution to form a super absorbent polymerparticle.

The surface crosslinking solution may comprise at least one surfacecrosslinking agent selected from the group consisting of a compoundhaving two or more epoxy rings and a compound having two or more hydroxygroups.

Preferably, the surface crosslinking solution may comprise both acompound having two or more epoxy rings and a compound having two ormore hydroxy groups. In this case, the surface crosslinking solutioncomprises a compound having two or more epoxy rings and a compoundhaving two or more hydroxy groups in a weight ratio of 1:1.1 to 1:5.

Examples of the compound having two or more epoxy rings include one ormore compounds selected from the group consisting of ethylene glycoldiglycidyl ether, polyethylene glycol diglycidyl ether, glycerolpolyglycidyl ether, propylene glycol diglycidyl ether, polypropyleneglycol diglycidyl ether, 1,4-butanediol diglycidyl ether,1,4-cyclohexanedimethanol diglycidyl ether, hexahydrophthalic anhydridediglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol Adiglycidyl ether, and N,N-diglycidylaniline. Preferably, ethylene glycoldiglycidyl ether is used.

Examples of the compound having two or more hydroxy groups include oneor more compounds selected from the group consisting of ethylene glycol,diethylene glycol, propylene glycol, triethylene glycol, tetraethyleneglycol, propane dial, dipropylene glycol, polypropylene glycol,glycerin, polyglycerin, butane diol, heptane dial, hexane dial,trimethylol propane, pentaerythritol, and sorbitol. Preferably,propylene glycol is used.

At this time, the surface crosslinking agent is preferably used in anamount of 1 part by weight or less based on 100 parts by weight of thebase polymer. Here, the amount of the surface crosslinking agent usedrefers to the total amount of the surface cross-linking agents when twoor more of the surface crosslinking agents are used. When the amount ofthe surface crosslinking agent used is more than 1 part by weight,excessive surface crosslinking may proceed and various physicalproperties, particularly dryness, of the super absorbent polymer may bedeteriorated. In addition, the surface crosslinking agent is preferablyused in an amount of 0.01 parts by weight or more, 0.02 parts by weightor more, 0.03 parts by weight or more, 0.04 parts by weight or more, or0.05 parts by weight or more based on 100 parts by weight of the basepolymer.

Moreover, the surface crosslinking solution may further comprises atleast one solvent selected from the group consisting of water, ethanol,ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol,propylene glycol, ethylene glycol monobutyl ether, propylene glycolmonomethyl ether, propylene glycol monomethyl ether acetate, methylethyl ketone, acetone, methyl amyl ketone, cyclohexanone,cyclopentanone, diethylene glycol monomethyl ether, diethylene glycolethylether, toluene, xylene, butyrolactone, carbitol, methyl cellosolveacetate and N,N-dimethylacetamide. Preferably, water can be used. Thesolvent can be used in an amount of 0.5 to 10 parts by weight based on100 parts by weight of the base polymer particle.

Furthermore, the surface crosslinking solution may further comprisealuminum sulfate. The aluminum sulfate may be contained in an amount of0.02 to 0.3 parts by weight based on 100 parts by weight of the basepolymer powder.

Further, the surface crosslinking solution may further comprise aninorganic filler. The inorganic filler may include silica, aluminumoxide, or silicate. The inorganic filler may be contained in an amountof 0.01 to 0.5 parts by weight based on 100 parts by weight of the basepolymer powder.

Further, the surface crosslinking solution may further comprise athickener. If the surface of the base polymer powder is furthercrosslinked in the presence of the thickener, it is possible to minimizethe deterioration of the physical properties even after thepulverization. Specifically, as the thickener, at least one selectedfrom a polysaccharide and a hydroxy-containing polymer may be used. Thepolysaccharide may be a gum type thickener, a cellulose type thickenerand the like. Specific examples of the gum type thickener includexanthan gum, arabic gum, karaya gum, tragacanth gum, ghatti gum, guargum, locust bean gum, and psyllium seed gum. Specific examples of thecellulose type thickener include hydroxypropylmethyl cellulose,carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose,hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethylmethylcellulose, hydroxymethylpropyl cellulose, hydroxyethylhydroxypropylcellulose, ethylhydroxyethyl cellulose, and methylhydroxypropylcellulose. Meanwhile, specific examples of the hydroxy-containingpolymer include polyethylene glycol, polyvinyl alcohol and the like.

Meanwhile, in order to perform the surface crosslinking, a method ofplacing the surface crosslinking solution and the base polymer into areaction tank and mixing them, a method of spraying a surfacecrosslinking solution onto the base polymer, a method in which the basepolymer and the surface crosslinking solution are continuously suppliedin a continuously operating mixer and mixed, or the like can be used.

In addition, the surface crosslinking may be carried out at atemperature of 100 to 250° C., and may be continuously performed afterthe drying and pulverizing step proceeding at a relatively hightemperature. At this time, the surface crosslinking reaction may becarried out for 1 to 120 minutes, or 1 to 100 minutes, or 10 to 60minutes. That is, in order to prevent a reduction in physical propertiesdue to damages of the polymer particles by excessive reaction whileinducing the minimal surface crosslinking reaction, the surfacemodification step may be performed under the above-described conditions.

Super Absorbent Resin

The super absorbent polymer produced by the preparation method of thepresent invention as described above has excellent dryness whilemaintaining excellent absorption performance.

Specifically, the super absorbent polymer according to the presentinvention has a centrifuge retention capacity (CRC) for a physiologicalsaline solution (0.9 wt % sodium chloride aqueous solution) for 30minutes of 30 g/g or more. The measurement method of the centrifugeretention capacity will be more specified in the following embodiments.Preferably, the centrifuge retention capacity is 30.5 g/g or more, or 31g/g or more. In addition, the higher the value of the centrifugeretention capacity is, the more excellent it is. Thus, the substantialupper limit is not restricted, but as an example, it is 35 g/g or less,34 g/g or less, or 33 g/g or less.

Further, the super absorbent polymer according to the present inventionhas a 6-hour dryness of 1.0 g or less. The 6-hour dryness refers to theamount of moisture present on the surface when 6 hours lapse after 2 gof the super absorbent polymer absorbs and swells a moisture and thelike. The concrete measurement method thereof will be further specifiedin the following embodiments. Preferably, the 6-hour dryness is 0.9 g orless, 0.8 g or less, or 0.7 or less. The smaller the value of thedryness is, the more excellent it is. Thus, the lower limit of the6-hour dryness is theoretically 0 g, but as an example, it is 0.1 g ormore, or 0.2 g or more.

Further, the super absorbent polymer according to the present inventionhas an absorption rate (vortex) of 35 seconds or less as measuredaccording to the measurement method of Vortex. The absorption raterefers to the time during which the vortex of the liquid disappears dueto fast absorption when the super absorbent polymer is added to aphysiological saline solution and stirred. This can define a fastabsorption rate of the super absorbent polymer. The measurement methodthereof will be more specified in the following embodiments. Preferably,the absorption rate is 34 seconds or less, or 33 seconds or less asmeasured according to the measurement method of Vortex. In addition, thesmaller the value of the absorption rate is, the more excellent it is.Thus, the lower limit of the absorption rate is theoretically 0 seconds,but as an example, it is 10 seconds or more, 20 seconds or more, or 25seconds or more.

Further, preferably, the super absorbent polymer according to thepresent invention has an absorbency under pressure (0.9 AUP) at 0.9 psifor a physiological saline solution (0.9 wt % sodium chloride aqueoussolution) for 1 hour of 7 g/g or more. The measurement method of theabsorbency under pressure will be more specified in the followingembodiments. Preferably, the 0.9 AUP is 7.5 g/g or more, or 8.0 g/g ormore. In addition, the higher the value of the absorbency under pressureit, the more excellent it is. Thus, the substantial upper limit is notrestricted, but as an example, it is 25 g/g or less, 24 g/g or less, or23 g/g or less.

Further, preferably, the super absorbent polymer according to thepresent invention has a liquid permeability of 5 seconds or more. Themethod for measuring the liquid permeability will be more specified inthe following embodiments. Preferably, the liquid permeability is 6seconds or more, 7 seconds or more, or 8 seconds or more. In addition,the higher the value of the liquid permeability is, the more excellentit is. Thus, the substantial upper limit is not restricted, but as anexample, it is 25 seconds or less, or 20 seconds or less.

In addition, in the super absorbent polymer according to the presentinvention, the ratio of particles having a particle diameter of 150 to850 μm is 90% or more.

Advantageous Effects

As described above, the super absorbent polymer according to the presentinvention has excellent dryness while maintaining excellent absorptionperformance, and thus is preferably used for hygienic materials such asdiapers and can exhibit excellent performance.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred examples are provided for better understanding ofthe invention. However, these Examples are given for illustrativepurposes only and are not intended to limit the scope of the presentinvention thereto.

EXAMPLE 1

100 g of acrylic acid, 0.35 g of polyethylene glycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.05 g of sodium persulfate (SPS) as athermal initiator, 0.06 g of sodium bicarbonate (SBC) as a foamingagent, 0.02 g of sodium dodecylsulfate as a surfactant, 83.3 g of 50%caustic soda (NaOH) and 89.8 g of water were mixed to prepare a monomeraqueous solution composition. The monomer aqueous solution compositionwas subjected to a thermal polymerization reaction to obtain apolymerized sheet. The polymerized sheet was taken out and cut into asize of 3 cm×3 cm. Then, the cut sheet was subjected to a choppingprocess using a meat chopper to prepare crumbs. Then, the crumbs weredried in an oven capable of shifting airflow up and down. The crumbswere uniformly dried by flowing hot air at 185° C. from the bottom tothe top for 15 minutes and from the top to the bottom for 15 minutes, sothat the dried product had a water content of 2% or less. After drying,the crumbs were pulverized using a pulverizer and then classified withan amplitude of 1.5 mm into three mesh sizes (combination of classifiedmeshes: #30/#50/#100). The respective classified particles (10%/72%/18%)were collected, and a polymer having a particle size of about 150 μm to850 μm was classified and obtained. The base polymer powder was obtainedby the above method.

Subsequently, to 100 parts by weight of the base polymer prepared, asurface crosslinking solution (4 parts by weight of water, 0.25 parts byweight of ethyleneglycol diglycidyl ether (EX-810), 0.3 parts by weightof propylene glycol (PG), 0.15 parts by weight of aluminum sulfate 18hydrate, and 0.1 part by weight of silica (Aerosil A200)) was uniformlymixed, and then subjected to a surface crosslinking reaction at 140° C.for 30 minutes. After completion of the surface treatment, the resultantproduct was sieved to obtain a super absorbent polymer having a particlediameter of 150 to 850 μm.

EXAMPLES 2 TO 9 AND COMPARATIVE EXAMPLES 1 TO 5

A super absorbent polymer was obtained in the same manner as in Example1, except that SPS, and the composition of the surface crosslinkingsolution were set as shown in Table 1 below.

EXPERIMENTAL EXAMPLE Evaluation of Physical Properties of SuperAbsorbent Polymer

The physical properties of the super absorbent polymer prepared inExamples and Comparative Examples were evaluated by the followingmethods.

(1) Extractable Contents (16 hr E/C)

The extractable contents were measured for the base polymer in thepreparation process of Examples and Comparative Examples by the samemethod as described in EDANA (European Disposables and NonwovensAssociation) recommended test method No. WSP 270.2.

Specifically, 1.0 g of the base polymer was put in 200 g of a 0.9 wt %NaCl solution, and then swollen for 16 hours while stirring at 500 rpm.The aqueous solution was filtered out through a filter paper. Thefiltered solution was primarily titrated to pH 10.0 with 0.1 N causticsoda solution, and then back-titrated to pH 2.7 with a 0.1 N hydrogenchloride solution. From the amount required during neutralization, theuncrosslinked polymer substance was calculated and measured as theextractable content.

(2) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity(CRC) by water absorption capacityunder a non-loading condition was measured for the super absorbentpolymers of Examples and Comparative Examples in accordance with EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.3.

Specifically, W₀ (g, about 0.2 g) of the super absorbent polymers ofExamples and Comparative Examples were uniformly put in a nonwovenfabric-made bag, followed by sealing. Then, the bag was immersed in aphysiological saline solution composed of 0.9 wt % aqueous sodiumchloride solution at room temperature. After 30 minutes, water wasremoved from the bag by centrifugation at 250 G for 3 minutes, and theweight W₂(g) of the bag was then measured. Further, the same procedurewas carried out without using the super absorbent polymer, and then theresultant weight W₁(g) was measured.

Using the respective weights thus obtained, CRC (g/g) was calculatedaccording to the following Mathematical Formula 1.

CRC(g/g)={[W₂(g)−W₁(g)−W₀(g)]/W₀(g)}  [Mathematical Formula 1]

in Mathematical Formula 1,

W₀(g) is an initial weight(g) of the super absorbent polymer, W₁(g) isthe weight of the device not including the super absorbent polymer,measured after immersing and absorbing the same into a physiologicalsaline solution for 30 minutes and then dehydrating the same by using acentrifuge at 250 G for 3 minutes, and W₂(g) is the weight of the deviceincluding the super absorbent polymer, measured after immersing andabsorbing the super absorbent polymer into a physiological salinesolution at room temperature for 30 minutes and then dehydrating thesame by using a centrifuge at 250 G for 3 minutes.

In addition, with respect to the respective base polymers produced inthe preparation process of Examples and Comparative Examples, CRC (BRCRC) was measured in the same manner as described above.

(3) Dryness

2 g of the super absorbent polymer was put in a 500 mL beaker and 200 mLof distilled water at 22 to 24° C. was added thereto. After the superabsorbent polymer was swollen, it was taken out and left at roomtemperature for 6 hours. Subsequently, after stacking five filter paperswith a diameter of 5 cm, the filter paper was placed on the superabsorbent polymer, and pressurized at 0.2 psi for 1 minute. The weight(dryness, g) of the distilled water absorbed by the filter pater wasmeasured.

(4) Absorption Rate (Vortex)

50 mL of a 0.9 wt % NaCl solution was put in a 100 mL beaker, and then 2g of each super absorbent polymer prepared in Examples and ComparativeExamples was added thereto while stirring at 600 rpm using a stirrer.Then, the vortex time was calculated by measuring the amount of timeuntil a vortex of the liquid caused by the stirring disappeared and asmooth surface was formed, and the result was shown as the vortexremoval time.

50 mL of a 0.9 wt % NaCl solution was put in a 100 mL beaker, to which amagnetic bar with a size of 30 mm (polygon type with a length of 30 mmand a thickness of 8 mm) was added. While stirring at 600 rpm using amagnetic stirrer, 2.0 g of the super absorbent polymers prepared inExamples and Comparative Examples were respectively added. Then, thevortex time was calculated by measuring the amount of time until avortex of the liquid caused by the stirring disappeared and a smoothsurface was formed, and the result was shown as the vortex removal time(absorption rate; vortex).

(5) Absorbency Under Pressure (AUP)

The absorbency under pressure (AUP) of the super absorbent polymers ofExamples and Comparative Examples was measured in accordance with EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 242.3.

Specifically, a 400 mesh stainless screen was installed at the bottom ofa plastic cylinder having an inner diameter of 60 mm. W₀(g, about 0.90g) of the super absorbent polymers obtained in Examples and ComparativeExamples were uniformly scattered on the stainless screen under acondition of a temperature of 23±2° C. of and a relative humidity of45%. Then, a piston capable of providing a load of 0.9 psi uniformly wasdesigned so that the outer diameter was slightly smaller than 60 mm andthus it could move freely up and down without any gap with the innerwall of the cylinder. At this time, the weight W₃(g) of the device wasmeasured.

A glass filter having a diameter of 125 mm and a thickness of 5 mm wasplaced in a Petri dish having a diameter of 150 mm, and a physiologicalsaline solution composed of 0.90 wt % sodium hydroxide aqueous solutionwas poured until the surface level became equal to the upper surface ofthe glass filter. Then, a sheet of filter paper having a diameter of 120mm was placed on the glass filter. The measuring device was placed onthe filter paper, so that the liquid was absorbed under load for onehour. After one hour, the measuring device was lifted and the weightW₄(g) was measured.

Using the respective weights thus obtained, AUP (g/g) was calculatedaccording to the following [Mathematical Formula 2.

AUP(g/g)=[W₄(g)−W₃(g)]/W₀(g)   [Mathematical Formula 2]

in Mathematical Formula 2,

W₀(g) is an initial weight (g) of the super absorbent polymer, W₃(g) isthe total sum of a weight of the super absorbent polymer and a weight ofthe device capable of providing a load to the super absorbent polymer,and W₄(g) is the total sum of a weight of the super absorbent polymerand a weight of the device capable of providing a load to the superabsorbent polymer, after absorbing a physiological saline solution tothe super absorbent polymer under a load (0.9 psi) for 1 hour.

(6) Liquid Permeability

In a state where a piston was introduced in a chromatography tube (F20mm), the liquid surface was displayed as a 20 ml mark line and a 40 mlmark line. Then, water was inversely introduced in a chromatography tubeso that bubbles are not generated between a glass filter and a cork atthe bottom of the chromatography tube, filling the tube forapproximately 10 ml, and the chromatography tube was washed 2 to 3 timeswith salt water and filled with 0.9% salt water up to 40 ml or greater.A piston was introduced in the chromatography tube, a valve at thebottom was opened, and then the time (B) taken for the liquid surfacedecreasing from a 40 ml mark line to a 20 ml mark line was recorded.

10 mL of salt water was left in the chromatography tube, to which0.2±0.0005 g of classified (30# to 50#) sample was added. Salt water wasadded thereto so that the salt water volume became 50 ml, and then theresult was left for 30 minutes. After that, a piston with a weight (0.3psi=106.26 g) was introduced in the chromatography tube, and the resultwas left for 1 minute. After opening a valve at the bottom of thechromatography tube, the time (T1) taken for the liquid surfacedecreasing from a 40 ml mark line to a 20 ml mark line was recorded.Thereby, the liquid permeability (time of T1 -B) was measured.

The results of the above measurement are shown in Table 1 below.

TABLE 1 Composition of Physical surface properties of Physicalproperties of super crosslinking agent base polymer absorbent polymerEX- BR 16 hr Liquid SPS Water 810 PG CRC EC CRC Dryness Vortex AULpermeability Unit ppmw phr phr phr g/g wt % g/g g sec g/g sec Ex. 1 5004 0.25 0.3 34.0 8.1 31.1 0.4 31 8.4 7 Ex. 2 500 4 0.25 0.6 34.0 8.1 30.70.3 32 9.1 8 Ex. 3 1000 4 0.25 0.3 35.1 10.7 32.3 0.4 33 7.2 8 Ex. 41000 4 0.25 0.6 35.1 10.7 32.0 0.3 30 8.1 8 Ex. 5 1500 4 0.25 0.3 34.911.6 31.8 0.4 31 9.7 11 Ex. 6 1500 4 0.25 0.6 34.9 11.6 31.5 0.2 30 10.710 Ex. 7 1500 4 0.25 1.0 34.9 11.6 30.2 0.5 33 13.1 16 Ex. 8 2000 4 0.250.3 34.8 12.5 31.9 0.7 30 9.1 14 Ex. 9 2000 4 0.25 0.6 34.8 12.5 31.40.6 32 9.6 16 Comparative 1500 4 0 0.25 34.9 11.6 35.5 3.0 31 6.5 0.7Ex. 1 Comparative 1500 4 0.25 2.0 34.9 11.6 29.0 2.7 31 14.9 23 Ex. 2Comparative 2500 4 0.25 0.6 34.3 15.5 33.2 2.5 35 7.1 4 Ex. 3Comparative 3000 4 0.25 0.6 34.5 19.2 34.2 2.6 37 8.6 8 Ex. 4Comparative 1500 4 0.25 0 34.9 11.6 31.4 0.6 30 10.0 9 Ex. 5

From the results of Table 1, it was confirmed that the super absorbentpolymers of Examples according to the present invention had excellentdryness while maintaining other physical properties equal to or higherthan those of the super absorbent polymers of Comparative Examples.

On the other hand, in the case of Comparative Examples, it was confirmedthat various physical properties were deteriorated according to the useamount of the thermal initiator or the composition of the surfacecrosslinking agent. Specifically, in the case of Comparative Examples 1and 2, since the use amount of propylene glycol as a surfacecrosslinking agent was small or large, the dryness was decreased. In thecase of Comparative Examples 3 and 4, the use amount of the thermalinitiator (SPS) was large and thus the dryness was decreased. Inaddition, in the case of Comparative Example 5, since propylene glycolas a surface crosslinking agent was not used, the dryness or liquidpermeability was relatively lowered as compared with those of Examples.

1. A method for producing a super absorbent polymer comprising the stepsof: crosslinking a water-soluble ethylenically unsaturated monomerhaving at least partially neutralized acidic groups in the presence ofan internal crosslinking agent and a thermal polymerization initiator;drying, pulverizing and classifying the hydrogel polymer to form a basepolymer power; and heat-treating and surface-crosslinking the basepolymer powder in the presence of a surface crosslinking solution toform a super absorbent polymer particle, wherein the thermalpolymerization initiator is used in an amount of 0.04 to 0.22 parts byweight based on 100 parts by weight of the ethylenically unsaturatedmonomer.
 2. The method of claim 1, wherein the thermal polymerizationinitiator is sodium persulfate, potassium persulfate, ammoniumpersulfate, 2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutylonitril,2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, or4,4-azobis-(4-cyanovaleric acid).
 3. The method of claim 1, wherein thesurface crosslinking solution comprises at least one surfacecrosslinking agent selected from the group consisting of a compoundhaving two or more epoxy rings, and a compound having two or morehydroxy groups.
 4. The method of claim 1, wherein the surfacecrosslinking solution comprises a compound having two or more epoxyrings and a compound having two or more hydroxy groups in a weight ratioof 1:1.1 to 1:5.
 5. The method of claim 3, wherein the compound havingtwo or more epoxy rings comprises one or more compounds selected fromthe group consisting of ethylene glycol diglycidyl ether, polyethyleneglycol diglycidyl ether, glycerol polyglycidyl ether, propylene glycoldiglycidyl ether, polypropylene glycol diglycidyl ether, 1,4-butanedioldiglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether,hexahydrophthalic anhydride diglycidyl ether, neopentyl glycoldiglycidyl ether, bisphenol A diglycidyl ether, andN,N-diglycidylaniline.
 6. The method of claim 3, wherein the compoundhaving two or more hydroxy groups comprises one or more compoundsselected from the group consisting of ethylene glycol, diethyleneglycol, propylene glycol, triethylene glycol, tetraethylene glycol,propane diol, dipropylene glycol, polypropylene glycol, glycerin,polyglycerin, butane diol, heptane diol, hexane diol, trimethylolpropane, pentaerythritol, and sorbitol.
 7. The method of claim 1,wherein the surface crosslinking solution further comprises aluminumsulfate, or an inorganic filler.
 8. The method of claim 1, wherein thesuper absorbent polymer has a centrifuge retention capacity (CRC) of 30g/g or more.
 9. The method of claim 1, wherein the super absorbentpolymer has a 6-hour dryness of 1.0 g or less.
 10. The method of claim1, wherein the super absorbent polymer has an absorption rate (vortex)of 35 seconds or less, as measured according to the measurement methodof Vortex.
 11. The method of claim 4, wherein the compound having two ormore epoxy rings comprises one or more compounds selected from the groupconsisting of ethylene glycol diglycidyl ether, polyethylene glycoldiglycidyl ether, glycerol polyglycidyl ether, propylene glycoldiglycidyl ether, polypropylene glycol diglycidyl ether, 1,4-butanedioldiglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether,hexahydrophthalic anhydride diglycidyl ether, neopentyl glycoldiglycidyl ether, bisphenol A diglycidyl ether, andN,N-diglycidylaniline.
 12. The method of claim 4, wherein the compoundhaving two or more hydroxy groups comprises one or more compoundsselected from the group consisting of ethylene glycol, diethyleneglycol, propylene glycol, triethylene glycol, tetraethylene glycol,propane diol, dipropylene glycol, polypropylene glycol, glycerin,polyglycerin, butane diol, heptane diol, hexane diol, trimethylolpropane, pentaerythritol, and sorbitol.